Cold and ultra-cold matter physics (e.g., optical traps, magneto-optical traps (MOTs), ion traps, laser cooling, and Bose-Einstein Condensates) has spurred demand for compact high vacuum (HV) and ultra-high vacuum (UHV, e.g., from about 10−9 torr to about 10−13 torr) systems. At these pressures, the mean free path of a gas molecule is on the order of 40 kilometers (km), so gas molecules typically collide with chamber walls many times before colliding with each other. For this reason, almost all interactions take place on chamber walls and other surfaces within a UHV chamber.
Several vacuum technologies may be used together to establish UHV. For example, a UHV cell may be baked at high temperatures to release particles prior to establishing UHV. Various pumping technologies can be used to establish UHV. However, UHV can degrade as particles are introduced intentionally (e.g., as part of an experiment) or unintentionally (e.g., by effusion from or diffusing through vacuum cell walls), so an active pumping technology is needed to maintain UHV. Sputter ion pumps are currently the most desirable and mature technology for actively maintaining UHV in a compact cell.
Herein, “sputter ion pump” (aka, “ion sputter pump”) refers to any system that removes mobile molecules (including single-atom molecules) from a local (incomplete) vacuum by: 1) ionizing the molecules to yield ions; 2) immobilizing the ions by sorbing (adsorbing or absorbing) them to a “getter” material; and 3) burying the ions with material sputtered from a cathode. Herein, “molecule” refers to the smallest particle in a chemical element or compound that has the chemical properties of that element or compound. A typical ion pump makes use of a Penning trap constituted by an electric field and a magnetic field. The electric field gives rise to free electrons at a cathode and accelerates them toward an anode. A cross product of the magnetic field with the electric current associated with the accelerating electrons produces a force orthogonal to the electron path. In a cylindrical geometry, this force diverts the electrons so that they form a swirling cloud.
The resulting cloud of swirling electrons ionizes incident molecules, which are then accelerated by the electric fields so that they impact surfaces of getter material to which the ions are adsorbed. In addition, some molecules, e.g., of hydrogen and noble gases, most significantly, helium, may be absorbed by the getter material. In a sputter ion pump, getter material may be liberated (“sputtered”) from the getter surface and then re-deposited, burying sorbed molecules and renewing the getter surface. In contrast to other common UHV pumps, such as turbo-molecular pumps and diffusion pumps, ion pumps have no moving parts and use no oil. They are therefore clean, need little maintenance, and produce little or no vibration.
Efforts are underway to make more compact UHV systems. UHV systems tend to be incorporated in other systems, the dimensions of which may scale with the size of the UHV system. A smaller UHV system will enable the incorporating systems to be more portable and less expensive. However, it is a challenge to maintain the ion-pump effectiveness at smaller dimensions. Therefore, it has become increasingly important to minimize the barriers to pumping effectiveness in compact UHV systems.